The adhesion of a compounded elastomer vulcanizate to metallic reinforcing elements, in rubber composites, has generally presented manufacturers with a major fabrication problem. Loss of rubber-to-metal adhesion results in separation of the elastomer system from the reinforcing element and ultimately to failure of the manufactured article. It is an object of the instant invention to improve the bonding of elastomer systems to metallic reinforcement elements, thus extending the service life of the article.
A number of fillers and various filler treatments to improve the adhesion of rubber compounds to metals have been used in the prior art. The most accepted state of the art materials, channel blacks were recognized as the filler of choice when compounding rubber vulcanizates for adhesion to metal. Channel blacks imparted moderately acidic properties to the rubber stocks, thus retarding the onset of cure and improving the adhesion of the rubber to metal. However, channel blacks are no longer available due to government regulations. Therefore, subsequent art has been directed toward fillers which will improve the adhesion performance over that of the standard furnace blacks available today. For example, PPG Industries publication Hi-Sil Silica Pigments Formulary, Volume II of Apr. 29, 1983 disclosed the use of precipitated silica as a filler for elastomer systems. Additionally, chemical treatments of furnace blacks such as are found in U.S. Pat. Nos. 3,676,255, 3,914,148, and 3,914,149 modify the characteristics of the furnace blacks to improve rubber-to-metal adhesion.
While the various fillers used in the prior art as channel black replacements do result in a limited cure retardation and improve rubber-to-metal adhesion over that of unmodified furnace blacks, these fillers also impart undesirable qualities to the stocks. Generally, these fillers increase the raw vulcanizate stiffness making the stock more difficult to process and lowering the process safety. The stiffer stocks also require more energy input to process them through mixers, mills, calenders, extruders, etc. Additionally, the alternate fillers moderately acidify the rubber stocks to a degree that they tend to be corrosive to the rubber process machinery. The higher cost of the various fillers is a further disadvantage.